Electrowinning may generally be described as an electrolytic process for recovery of a metal using an aqueous solution of an acid and a metal salt as an electrolyte and an insoluble electrode as an anode to deposit a highly pure metal on a cathode. This method is used to produce a variety of highly pure metals, such as nickel, cobalt, copper, zinc, etc. Typically, electrowinning is carried out in large, open-topped tanks provided with a plurality of flat electrode plates suspended into the electrolyte solution from the top of the tank. A number of such tanks are usually housed in a building known as a tank house.
During the electrolysis reaction, oxygen is liberated at the anode. The oxygen is produced in the form of tiny bubbles which rise to the surface of the electrolyte and burst. These bubbles, comprising a thin layer of electrolyte, emit a mist of near electrolyte composition to the atmosphere surrounding the electrolytic cell when they burst. The composition of the mist is dependent on the composition of the electrolyte, and typically contains sulfuric acid and metal salts.
In view of the fact that electrowinning is typically carried out in enclosed tank houses, the generation of electrolyte mist causes serious concerns in relation to worker safety as well as corrosion of equipment. Various methods have been employed to either contain or inhibit the generation of mist by electrolytic cells. However, none of these methods has proved to be satisfactory.
One common method to deal with electrolyte mist is to provide a powerful ventilation unit to remove contaminated air from the tank house, along with scrubbers to remove contaminants from the air before it is either recirculated or released into the environment. This method has proved to be wasteful of energy and not very effective.
Attenuation of mist production has been attempted by providing layers of floating plastic balls, beads, rods, discs, etc. in the electrolyte in an attempt to provide a surface on which the mist from the bursting gas bubbles may be collected and drain back into the electrolyte bath. Another attempted solution has been to add a surfactant to the electrolyte bath, thereby reducing its surface tension and reducing the intensity of mist breakout. However, neither of these methods has proved to be satisfactory.
Another attempted solution to mist generation is disclosed by U.S. Pat. No. 4,075,069 to Shinohara et al. This patent provides an inert woven fabric screen over an electrode plate from a position above the level of the electrolyte to the bottom of the electrode. The size of the openings in the screen are chosen so that bubbles of gas will not substantially pass through the screen, but rather will coalesce into larger bubbles as they pass upwardly along the surface of the electrode. When these larger bubbles burst at the surface of the electrolyte, they apparently generate less electrolyte mist than if the bubbles were not coalesced. Bubble coalescence has also been used by others to reduce the generation of electrolyte mist, for example see U.S. Pat. Nos. 4,584,082 and 4,668,353. However, bubble coalescence permits some mist to escape into the atmosphere, and is therefore of limited use. In addition, the size of the openings in the mesh in Shinohara et al. are believed to be sufficiently large as to allow some of the gas to be swept away from the face of the anode by modem cell flow systems, which would further reduce its capacity to reduce mist.